Generally, photolithography refers to a method of utilizing light to transfer a design onto a printed circuit boards or a wafer's surface. For example, in one operation, a semiconductor substrate is covered with a photoresist (light sensitive) layer. The photoresist layer is then masked such that the desired design is either the only part of the photoresist layer covered by the mask or not covered by the mask. The entire layer (e.g., photoresist layer and the mask layer) is then exposed to a single burst from a light source with enough energy to harden, or cure, the unmasked portions of the photoresist layer.
In the positive photolithography process, the portion of the resist that has been exposed to light is removed utilizing an etching method such as an acid bath, hot ions, or the like. In the negative photolithography process, the portion of the resist that has not been exposed to light is removed. That resist removal process is called “development”. After development, the remaining resist constitute the wanted mask, and the underlying substrate is able to be etched
However, there is one significant problem with the present state of photolithography and the growing desire for smaller designs. That is, the limitation in the size of the design that can be formed using the standard photolithographic methods. Presently, standard photolithographic process is limited to approximately the size of a wavelength of light. Thus, the ability to design tens or even few hundreds of nanometer size features using standard photolithographic techniques is practically impossible.
In order to overcome the shortcomings of the standard photolithographic process, more costly and time consuming methods such as electron beam lithography are utilized. In general, electron beam lithography refers to a method of significantly reducing the lithographic pattern size by using a beam of electrons that can be focused to much smaller areas than light.
For example, the beam width can be as small as approximately 10's of nanometers. However, this form of lithography is deleteriously slow. For example, the focused beam of electrons must be written across the surface in a serial fashion. Thus, the pattern generation performed by the electron beam lithography process is extremely slow when compared with the parallel exposure technique of photolithography.